The present invention pertains to a method and apparatus for evaluating the quality of a cable line. In particular, the present invention relates to a method and apparatus for evaluation of the quality of a cable line employing a technique for efficiently and precisely evaluating the quality of a cable line for transmitting a digital modulation signal.
A CATV network achieved to overcome difficulty in audio and visual broadcasting, for example, is configured as shown in FIG. 6.
That is, in this CATV network, one coaxial cable 2 connected to a head end 1 of a center is branched in a tree shape in order to distribute a broadcasting signal to a number of subscribers with a small number of cables.
Image receiving terminals of subscribers can be connected to subscriber"" terminals 2a, 2a, . . . of their tip ends (reference numeral 2 in the figure denotes a tap-off that is a branch node).
This CATV network has been initially utilized as a unidirectional network for distributing a broadcasting signal to each subscriber.
Hence, in such CATV network as well, in recent years, the construction of a bidirectional network has been developed so that data can be transmitted from a subscriber to the center by utilizing a dummy frequency bandwidth as a upstream line.
In the meantime, in the CATV network having its tree shaped transmission path as described above, a life noise xe2x80x98nxe2x80x99 of the upstream line frequency bandwidth generated by the home electric equipment of each subscriber (for example, noise generated by turning ON/OFF micro-oven or refrigerator compressor or the like) is unavoidably superimposed over one coaxial cable 2.
The degraded quality of a cable line caused by a noise signal N superimposed over this coaxial cable 2 being transmitted to a head end 1 is problematic.
That is, this noise signal N is called an ingress noise, and its level or frequency-distribution greatly differs depending on network environment, and varies with an elapse of time, which is the primary factor that prevents the construction of a bidirectional CATV network.
Because of this, in the case where the existing CATV network is constructed as a bidirectional network or in the case where a bidirectional CATV network is newly constructed, it is required to initially investigate whether or not an upstream line can be laid to prevent this ingress noise.
In order to investigate this, conventionally, as shown in FIG. 6, a spectrum analyzer 5 (instrument capable of monitoring the noise level of each frequency) has been connected to the head end terminal of the coaxial cable 2 so as to monitor the ingress noise N of the upstream line over a long period of time.
Then, statistic processing has been performed for the monitoring result, and it has been judged whether or not the upstream line is available, namely, whether or not an S/N corresponding to a predetermined error rate can be fully ensured by predetermined limited signal power against the ingress noise N.
In the case where it is judged that an upstream line is available, a test carrier xe2x80x98Stxe2x80x99 modulated by a digital signal of its predetermined pattern is inputted from the subscriber""s terminal 2a of the coaxial cable 2, as shown in FIG. 7.
This test carrier xe2x80x98Stxe2x80x99 is received and demodulated at the head end 1 together with the ingress signal N.
At the head end 1, for example, as in a code error detecting system disclosed in the U.S. Pat. No. 5,390,199, a demodulated signal is compared in bits by using a pseudo random signal, the error rate of this modulated signal is measured, and the line quality between each subscriber""s terminal 2a and the head end 1 is individually evaluated in a state similar to actual operation state.
However, in the method of judging the availability based on the level of the ingress noise as described previously, there is a possibility of mistaken judgment.
This is because the ingress noise N is likely to include a periodic noise component or a pulse-like noise component as well as white color noise free of self correlation.
That is, a correlation between the S/N of a signal transmitted by the coaxial cable 2 and the error rate thereof cannot be obtained due to a number of such noise components.
Namely, assuming that a sufficient S/N can be ensured, even if the test carrier xe2x80x98Stxe2x80x99 is actually inputted from the subscriber""s terminal 2a, as shown in FIG. 7, a desired error rate cannot be obtained frequently, thus making it impossible to move to the next test operation stage.
Therefore, it is considered that the test carrier xe2x80x98Stxe2x80x99 is initially inputted from the subscriber""s terminal 2a, and the error rate is measured without performing monitoring for the ingress noise, as shown in FIG. 7.
Hence, even in the case where there is no possibility that the upstream line is available, measurement of all the subscriber""s terminals are wastefully performed, and the efficiency of cable line quality evaluation is very poor.
It is an object of the present invention to provide a method and apparatus for evaluating the quality of a cable line that solves the aforementioned problem with the prior art, and makes it possible to efficiently and precisely grasp whether or not the line is available in quality.
According to one aspect of the present invention, there is provided a cable line quality evaluating method for evaluating a quality of a cable line for transmitting a digital modulation signal in a bidirectional manner, the method comprising the steps of:
extracting a noise signal of an upstream line from one of a cable line connection point between a head end of a cable line evaluated in the quality and the cable line evaluated in the quality and a connection point between a tap-off and the cable line evaluated in the quality;
generating a pseudo random signal;
modulating a carrier signal of its predetermined frequency by the pseudo random signal to output the carrier signal as a test carrier;
outputting an output signal obtained by adding a noise signal of the upstream line and the test carrier;
selectively receiving the signal of its predetermined frequency from the output signal and demodulating the signal; and
comparing the demodulated signal with the pseudo random signal in bits, and then measuring a bit error rate.
According to another aspect of the present invention, there is provided a cable line quality evaluating apparatus for evaluating a quality of a cable line for transmitting a digital modulation signal in a bidirectional manner, the apparatus comprising:
pseudo random signal generating means for generating a pseudo random signal (21);
test carrier generating means (22) for modulating a carrier signal of its predetermined frequency by means of a pseudo random signal from the pseudo random signal generating means, and outputting the modulated signal as a test carrier;
signal adding means (24) connected to one end of the cable line, the signal adding means adding a noise signal on an upstream line of the cable line and a test carrier outputted from the test carrier generating means;
signal receiving means (25) for selectively receiving the signal of its predetermined frequency from an output signal of the signal adding means, and modulating the signal; and
error measuring means (26) for comparing a signal demodulated by the signal receiving means with a pseudo random signal outputted from the pseudo random signal generating means in bits, thereby measuring a bit error rate.